I. Field
The present invention pertains to communications, and more particularly to a method and apparatus for combined puncturing and repeating of code symbols in a communications system.
II. Background
In a typical digital communications system, data is processed, modulated, and conditioned at a transmitter unit to generate a modulated signal that is then transmitted to one or more receiver units. The data processing may include, for example, formatting the data into a particular frame format, encoding the formatted data with a particular coding scheme to provide error detection and/or correction at the receiver unit, puncturing (i.e., deleting) some of the code symbols to fit within a particular frame size, channelizing (i.e., covering) the encoded data, and spreading the channelized data over the system bandwidth. The data processing is typically defined by the system or standard being implemented.
At the receiver unit, the transmitted signal is received, conditioned, demodulated, and digitally processed to recover the transmitted data. The processing at the receiver unit is complementary to that performed at the transmitter unit and may include, for example, despreading the received samples, decovering the despread samples, inserting xe2x80x9cerasuresxe2x80x9d in place of punctured symbols, and decoding the symbols to recover the transmitted data.
A digital communications system typically employs a convolutional code or a Turbo code to provide error correction capability at the receiver unit. The ability to correct transmission errors enhances the reliability of a data transmission. Conventionally, convolutional and Turbo coding is performed using a particular polynomial generator matrix that generates a particular number of code symbols (e.g., 2, 3, or more code symbols) for each input data bit. For example, a rate 1/2 encoder generates two code symbols for each data bit.
A multiple access communications system typically transmits data in frames or packets of predetermined sizes to allow for efficient sharing of system resources among active users. For example, some communications systems support frame sizes that are multiple times a basic frame size (e.g., 768xc2x7K bits, where K=1, 2, . . . ). For efficiency, some communications systems also support multiple data rates. Depending on a number of factors, a variable number of data bits (i.e., X) may be provided to the encoder, which then generates a corresponding number of code symbols (e.g., 2X).
In certain instances, the number of code symbols generated is not exactly equal to the capacity of the frame. Symbol repetition and puncturing are then used to fit the generated code symbols into a frame of a particular size. For example, if the number of code symbols is less than the frame capacity, some or all of the code symbols may be repeated (i.e., duplicated) a particular number of times. Conversely or additionally after the symbol repetition, if the number of code symbols is greater than the frame capacity, some of the code symbols may be deleted (i.e., punctured).
One conventional method for puncturing code symbols is to systematically puncture one symbol out of every Dth symbols until the required number of symbol punctures is achieved. The remaining symbols are then sent unmodified. In certain situations, this method can puncture symbols unevenly throughout an entire frame, which results in more symbols being punctured in one portion of the frame and less or no symbols being punctured in some other portion of the frame. When symbols are unevenly punctured, performance may be compromised.
As can be seen, techniques that can be used to puncture symbols in a manner to provide improved performance are highly desirable. Thus, there is a need for an improved technique for symbol puncturing and repetition.
The presently disclosed method and apparatus are directed to an improved technique for symbol puncturing and repetition. Accordingly, in one aspect of the invention, a method for combined repeating and puncturing of symbols in a communications system is provided. The method advantageously includes (a) receiving a first number of symbols L to be fitted into a frame having a capacity of N symbols; (b) initializing to zero an accumulator value and a symbol index value; (c) if the accumulator value is less than N, increasing the accumulator value by L and repeating in the frame a symbol from a location in the first number of symbols corresponding to the symbol index value, the increasing and repeating being performed until the accumulator value is not less than N; (d) if the accumulator value is greater than or equal to N, increasing the symbol index value by one and decreasing the accumulator value by N; and (e) repeating (c)-(d) until the symbol index value is greater than or equal to L.